Poly(ethylene terephthalate) resins are commonly referred to in the industry as “PET” even through they may and often do contain minor amounts of additional components. PET is widely used to manufacture containers for juice, water, carbonated soft drinks (“CSD”) and the like. PET is used for these purposes due to its generally excellent combination of mechanical and gas barrier properties.
The PET containers referred to herein are stretch blow molded containers. As would be recognized by one of ordinary skill in the art, stretch blow molded PET containers are manufactured by first preparing an injection molded preform from PET resin. The PET resin is injected into the preform mold that is of a certain configuration. In prior art methods of container manufacturer, configuration of the preform is dictated by the final bottle size and the properties of the polymer being used to prepare the container. After preparation of the preform, the preform is blow molded to provide a stretch blow molded container.
PET containers must conform to fairly rigid specifications, especially when used to contain and store carbonated beverages in warm climates and/or in the summer months. Under such conditions, the containers often undergo thermal expansion, commonly referred to in the industry as “creep”, caused by the high pressure in the container at high temperature. The expansion increases the space between the PET molecules in the side wall of the container thus allowing for CO2 to escape through the side wall faster than under normal conditions. Expansion also increases the head space of the container, which allows carbonation to escape from the beverage into the headspace area. Regardless of how carbonation is released from the beverage while enclosed in a container, loss of carbonation is undesirable because the beverage will taste “flat” when this occurs. Creep increases the interior space in the container which, in turn, reduces the height of the beverage in the container. This reduced height can translate into a perception by the consumer that the container is not completely full and, as such, perception of product quality is reduced.
PET container performance is also relevant in regards to sidewall strength. In storage and transport, filled PET containers are normally stacked with several layers of filled containers on top of each other. This causes significant vertical stress on the container which is manifested in large part against the sidewalls. If there is not sufficient sidewall strength or top load in the PET container, the bottle can collapse in storage or in use.
Moreover, consumer perception of container quality is manifested in the feel of the container when it is being held. When consumer hold a container and squeeze the container, the contain sidewall will deform. If sidewall deflection is too high, the container will feel too soft; and consumers relate this to a poor quality of products, even though the products are of the same quality as compared with products packed in a stiffer package.
One of ordinary skill in the art would recognize that it is desirable to reduce the amount of PET used in the preparation of PET containers for cost reduction. Lower weight PET containers result in lower material costs, less energy usage during the manufacturing process and lower transport costs. Lighter weighted containers also provide less solid waste and have less negative environmental impact. However, with reducing the amount of PET per container the desired properties mentioned above are also sacrificed, thus achieving a balance between source reduction and performance is difficult to achieve.
Prior art methods of reducing the weight of PET containers generally focus on reduction of the amount of polymer used to prepare the container. The weight of the container can be reduced to an amount that is shown through performance testing to not dramatically sacrifice performance of the containers in use, although some deterioration in container performance are seen with prior art methods of lightweighting where no barrier coating is used. Generally, the above-described container properties are directly related to the amount of PET resin used to prepare the container. In prior art methods of light weighting containers, lower amounts of PET resin used will result in thinner-walled finished containers and will consequently result in lower barrier and strength properties in the finished container. Thus, the tension between maximizing the performance of PET containers while attempting to reduce the weight of PET containers remains a concern, especially in warmer climates.
Energy consumption during the container manufacturing process is directly related to the thickness of the preform, because in a thicker preform there is more polymer mass present to heat and cool. Therefore, one method to reduce energy costs associated with preparation of PET containers is to lightweight the preform by reducing the thickness of the preform. Prior art methods for doing so involve making a core change or a cavity change to the preform design. A core change increases the inside diameter of the preform by hollowing out a portion of the inner wall of the preform. A cavity change does not affect the inner diameter but rather removes a portion of the outer wall of the preform. However, the thickness of the preform is related to, in part, the natural stretch ratio of the polymer being used to prepare the preform. That is, the natural stretch ratio of the polymer determines the stretch ratio of the preform, which is a function of the preform inner diameter correlating to thickness of the preform and height of the preform below the finish. The preform is designed to have a preform stretch ratio that is somewhat higher than the natural stretch ratio of the polymer, thus maximizing the performance of the PET resin by stretching the PET resin beyond its strain hardening point optimizing crystallization and orientation to create haze-free or substantially haze-free containers with acceptable mechanical performance. Increasing the inner diameter of a preform lowers the preform stretch ratio, which affects the final container properties by not maximizing the stretch of the PET resin. Therefore, it has been understood in the prior art that use of PET resin which has a natural stretch ratio typically in the range of about 13 to 16 has limitations in reducing energy costs in the container manufacturing process because the thickness of the preform cannot be effectively reduced.
One prior art method, which has been used to improve container quality, improve the productivity through reduced cycle time by using thinner walled preforms, and lessen energy consumption in manufacture, is to lower the stretch ratio of the polymer allowing for a reduced stretch ratio of the preform. Attempts have been made to lower the stretch ratio of the polymer by modification of the PET resin itself. This has been achieved by increasing the molecular weight or intrinsic viscosity (IV) of the PET resin because higher IV PET resins result in polymers with lower natural stretch ratios. However, when the IV of the PET resin is increased, the polymer will have higher melt viscosity. When higher melt viscosity is present, a higher melt temperature must be used to process the polymer. This results in more energy usage and also more potential for polymer degradation during processing. The higher melt temperature also requires longer cycle time during injection molding. These negative properties resulting from this method to lower the stretch ratio of the polymer thus outweigh any benefits described above in reducing the preform wall thickness.
Lowering of the polymer stretch ratio can also be accomplished by addition of long chain branching. However, like modifying the PET resin IV, this method also increases the melt viscosity of PET and caused the same problem of the high IV polymer. Thus, this method is not desirable.
In view of the above, it would be desirable to develop a preform design that does not result in higher energy consumption during processing. Still further, it would be desirable to develop a preform design that provides good mechanical properties in a finished stretch blow molded container such as, low thermal expansion, good sidewall rigidity and haze-free or substantially haze free containers. Still further, it would be desirable to reduce the energy consumption during injection molding the preform and, therefore, the container manufacturing process. The present invention meets these objectives.